Info

11-19,13-12

7. Estimate subsystem weight and power

Use analogy with existing systems

Table 11-26

8. Document reasons for selection

Important to document assumptions

N/A

\\22 Designing the TT&C Subsystem

Table 11-23 lists parameters for the TT&C subsystem we should specify and monitor in addition to the system-level requirements in Table 11-19.

TABLE 11 -23. Design Parameters for the TT&C Subsystem. Below are design parameters for the TT&C subsystem that are not typically specified at the system level. (See Table 11-19 for system-level parameters.)

Parameter

Comment

Antenna Sidelobe Levels

Design to minimize. Sldeiobes degrade the antenna's directionality. Very high sidelobes may interfere with other antennas and receivers on the satellite. High sidelobe levels also affect security by making detection of signals more likely.

Polarization

Polarizations can be circular (right or left), or linear (horizontal or vertical). To decrease signal loss In the link, the polarizations need to be compatible. For example, the satellite antenna and ground station must both have right-circular polarization.

Frequency Stability

When we need to acquire the signal quickly, the receiver frequency must be known and stable. Thus, we specify the original receiver frequency's set point, short-term stability, temperature stability, and aging stability so we can acquire the uplink signal with little uncertainty.

Capture and

Tracking

Range

The capture range is the band of frequencies over which the uplink-carrier signal can drift from the receiver's best-lock frequency, so the receiver will still lock to the uplink signal. The trading range is the band of frequencies the receiver will follow while locked to a sweeping, uplink-carrier signal without losing lock. Typically the capture range is 1 % of the tracking range.

Diplexer Isolation

The diplexer allows us to use the same antenna for transmitting and receiving. The diplexer isolates the transmitter from the receiver. A diplexer with low isolation may require a band-reject filter between the transmitter and the diplexer.

Coupling Between Antennas

Signal and noise coupling between a transmitting antenna and a receiving antenna may cause the receiving antenna to lock onto a frequency coming from the transmitting antenna's transmitter. More commonly, broadband noise from the transmitting antenna may couple over to the receiving antenna and raise the noise floor of the receiving antenna's receiver, reducing the signal-to-noise ratio.

As Table 11-24 shows, selection criteria for TT&C subsystems fall into three categories: performance, compatibility, and other. Performance is the most important selection criterion. This subsystem's hardware must meet specifications of minimum performance to close the communication link with an acceptable signal-to-noise ratio. The Bit Error Rate (BER) is a Figure of Merit for the digital part of the communication link. It is the probability that a bit sent over the communication link will be received incorrectly. We typically specify this rate to be 1 x 10-6 for the command uplink and 1 x 10~s for the telemetry downlink, depending on the nature of the data. To achieve this Tate, the system must meet certain technical specifications: RF power output for the transmitter, receiver-noise figure, stable oscillator frequency, and the TT&C subsystem's front-end losses and antenna gains.

Compatibility is an important selection criterion when the TT&C subsystem must communicate with existing systems. If the TT&C subsystem must talk to the ground stations in the Space Ground Link System (SGLS), then the transponder must be compatible with SGLS. Likewise, if it must talk to the Tracking and Data Relay Satellite System (TDRSS), the cross-link transponder must be compatible with TDRSS.

TABLE 11-24. Selection Criteria for the TT&C Subsystem. Selection criteria at the subsystem level fall Into three broad categories: performance, compatibility, and other. Chapter 13, Table 13-1, shows the selection process for communications architectures. (Courtesy of TRW)

Category

Criteria

Comments

• Power (RF and dc)

} See Table 11-26

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